Cleanly Combine Two Landscapes
You can combine two landscape using the bridge tool.


Select one of the landscape in object mode and move it away from the other slightly.


Select both landscapes in object mode
Join the two object together, Ctrl J


In edit mode select both edge loops between the landscapes.


bridge them together using the bridge tool, W -> Bridge Edge Loops


Number of Cuts: adjust this so that the generated polygons look square
Interpolation: Blend Surface, has the best look for bridging terrain
Smoothness: adjust this to your liking







Final Result